View Full Version : Fan Static Pressure

14-09-2003, 04:09 AM
How to classified the term "Total Static Pressure", "External Static Pressure" & "Velocity Pressure" and how to use to calculate the air duct pressure drop, air grille, weatherproof louvre, air filters pressure drop.

What is a differential pressure meter.how to install to read the differential between the room.

04-10-2003, 06:38 AM
Perhaps I can help just a bit. Grilles, louvers and filters normally are given an individual "pressure drop" by their respective manufacturers. For example: a given filter might be rated by its manufacturer for 0.2" W.C. pressure drop at 300 feet per minute air velocity. The same will be true for louvers, etc. A fan or air hanlder manufacturer will rate the blower at a variety of "external static pressures" to deliver a given air flow rate. This will be in the form of a table or graph.
When you assemble all the data from the manufacturers about their respective devices, add them all together. Then calculate the additional static pressure drop from ducts, etc. to get the total air pressure drop seen by the air handler or fan.
I use a Dwyer "Magnehelic" gauge to read differential pressure. It has fittings for two hoses. One will go to the room and the other to the outside or another room or space. The differential pressure is then read directly on the gauge.

04-10-2003, 04:24 PM
thank you, Bruce

I will try the calculation, as my project is near completion.

05-10-2003, 06:43 AM
Not sure what you need to know if the project is near completion, but the terms you inquired about can be expained as follows:

In any fluid stream, there are two pressures present. The velocity pressure is the pressure exerted by the flowing mass in the direction of flow but it can't be measured alone. The second pressure is the static pressure which acts in all directions, and is the pressure that is exerted by the fluid on the walls of the duct or pipe.

The measurements need to be taken in a long enough section of tube or duct so that the flow stream is smooth, not turbulent.

If a sensing tube is placed in the stream parallel to the flow with an opening facing upstream, this probe will register the total pressure of the system, both velocity pressure and static pressure.

If another probe is inserted at right angles to the flow, this will register only the static pressure.

If the two probes are connected to a device (a manometer) so that the pressures oppose one another, the effect is to subtract the static pressure from the total pressure picked up by the first probe and the resultant reading gives the velocity pressure.